1. Field of the Invention
The present invention relates generally to the field of wireless transmitters and receivers and particularly to a method and apparatus for antenna beamforming of a multi-input-multi-output (MIMO) transmitter and receiver using orthogonal frequency division multiplexing (OFDM).
2. Description of the Prior Art
Personal devices, such as computers, phones, personal digital assistants and the like have gained wide popularity in recent years. As technology improves, these devices have become increasingly smaller in size and highly portable. In fact, wireless, portable devices of various types now commonly communicate with one another allowing users flexibility of use and facilitating data, voice and audio communication. To this end, networking of mobile or portable and wireless devices is required.
With regard to the wireless networking of personal devices, a particular modem, namely modems adapted to the up-coming IEEE 802.11n industry standard, are anticipated to be commonly employed. That is, an array of antennas is placed inside or nearby the personal device and a radio frequency (RF) semiconductor device receives signal or data through the array and an analog-to-digital converter, typically located within the personal device, converts the received signal to baseband range. Thereafter, a baseband processor is employed to process and decode the received signal to the point of extracting raw data, which may be files transferred remotely and wireless, from another personal device or similar equipment with the use of a transmitter within the transmitting PC.
To do so, pointing of the array of antennas, which is essentially multiple antennas, hence the name multi-input-multi-output (MIMO), to the desired location to maximize reception and transmission quality is an issue. For example, data or information rate throughput, signal reception and link range are improved. The latest IEEE802.11n standard currently being developed includes advanced multi-antenna techniques in order to process parallel data streams simultaneously in order to increase throughput capability, and improve link quality by “smartly” transmitting and receiving the RF signals.
There are two basic types of beamforming specified in the current draft (2.0) 802.11n standard: explicit and implicit. For explicit beamforming the receiver measures the channel between the transmitter and receiver and sends this information, called channel state information or CSI, back to the transmitter. The transmitter can then use the channel information to calculate the best transmit “paths” or “directions” for that particular client for transmitting future packets. Using the CSI in this way is sometimes referred to as beamforming. While this method provides a direct measure of the channel for beamforming, it requires CSI to be sent over the link resulting in network overhead that can lead to reduced overall throughput.
The second basic method of beamforming, implicit beamforming, does not require CSI to be sent back to the transmitter on a packet-by-packet basis. Instead the implicit method relies on the principle of channel reciprocity. Channel reciprocity assumes that the upstream and downstream channels are essentially the same (to within a transpose operation), so that the receiver can use the measured channel information to beamform packets of information back to the transmitter. In this way, no explicit CSI is required to be sent over the link, thereby eliminating network overhead. The downside of implicit beamforming is that it requires a calibration procedure between the transmitter and receiver to ensure that reciprocity is achieved. The calibration procedure requires complex coordination between the access point (AP) and clients in which large amounts of CSI are periodically exchanged. An AP is a device that is wirelessly transmitting or receiving within a network of devices.
The problem with the methods currently proposed to implement calibration for implicit beamforming is that they are cumbersome, complex and inefficient. For example, for each sub-carrier, which are 56 or 118, in the current standard, an FFT is performed, the results of which are transmitted from a personal device to another device thereby significantly adversely effecting throughout and efficiency and over-complicating the problem.
Thus, while methods are currently proposed for implicit beamforming, these methods do not eliminate the need for time consuming and complex calibration exchanges: in implicit beamforming, as defined by the IEEE802.11n D2.0 Standard, a calibration exchange is required between a transmitter and receiver pair. This is a complicated exchange that requires CSI information per sub-carrier be passed in both directions (bi-directionally) between the receiver and transmitter. The network throughput performance will degrade because this calibration step is a time-consuming process that needs to be repeated whenever channel conditions change, and requires a large amount of data to be sent over the network.
Moreover, complexity and interoperability issues arise because the implicit beamforming calibration currently requires a complicated exchange, there is a risk that solutions from different vendors will not work together due to small differences in implementation or data formatting. Further, there is a substantial processing that needs to be implemented in hardware/firmware to implement this calibration exchange.
The calibration as described in the standard will not compensate for non-linear front-end impairments, such as IQ (in-phase and quadrature) non-orthogonality.
In light of the foregoing, it is desirable to develop a transceiver for receiving and transmitting signals in conformance with the 802.11n standard with an improved calibration for implicit beamforming.